Shift registers are widely used in, for instance, data signal driving circuits and scanning signal driving circuits in image display devices, so as to take timings for sampling data signals from image signals, and to produce a scanning signal to be applied to each signal line. Furthermore, in image display devices in which display sections or imaging sections are provided so as to be turned around, mirror images obtained by vertically or horizontally inverting images need be displayed depending on directions of the display section or the imaging section. Therefore two-way shift registers are used as the foregoing shift registers in the foregoing devices. In this case, upon switching of the shift direction, a direction for scanning images is inverted. Consequently, mirror images can be displayed without recording image signals in pixels.
On the other hand, consumed electric power of an electronic circuit increases proportionally to a product of a frequency, a load capacitance, and a square of a voltage. Therefore, in a circuit connected with an image display device, for instance, a circuit for producing image signals for an image display device, or in an image display device, a driving voltage tends to be set further lower so that the power consumption should be suppressed.
In circuits using polycrystalline silicon thin film transistors so as to ensure a large display area, for instance, in pixels, in data signal line driving circuits, or in scanning signal line driving circuits, differences in threshold voltages between substrates or in one and the same substrate are occasionally as much as several volts. Therefore, it cannot be considered that sufficient decrease in the driving voltage has been achieved. However, the driving voltage in a circuit using monocrystalline silicon transistors such as the foregoing image signal generating circuit are often set to 5V, or 3.3V, or lower than that. For this reason, in the case where input signals lower than the driving voltage of the shift register are applied, the shift register is provided with a level shifter for boosting the voltage of the input signal.
More specifically, in the case where a start signal SP with an amplitude of about 5V is applied to a conventional shift register 101 as described above, as shown in FIG. 9, for example, a level shifter 103 boosts the start signal SP to a level of a driving voltage (15V) of the shift register 101. An output of the level shifter 103 is applied to both a flip-flop F1 on one end of a shift register section 102 and a flip-flop Fn on the other end of the same. The shift register section 102 shifts the start signal SP in a direction corresponding to a switching signal L/R, in synchronization with a clock signal CK.
In the foregoing conventional shift register 101, however, the start signal SP is transferred to the flip-flops F1 and Fn after being level-shifted, and therefore, the following problem arises: as the distance between the flip-flops F1 and Fn increases, the transmission distance increases, and hence, the consumed power increases.
More specifically, as the transmission distance increases, the capacitance of the transmission-use signal line increases, thereby requiring a greater driving power of the level shifter 103, and hence, causing the consumed electric power to increase. Furthermore, in the case where the driving power of the level shifter 103 is in sufficient as in the case where the foregoing driving circuit including the level shifter 103 is formed using polycrystalline silicon thin film transistors, a buffer 104 is required to be provided between the level shifter 103 the flip-flop Fn, as shown with dotted lines in the figure, so that a non-distorted waveform is transferred. Consequently, further more electric power is consumed.
Recently, the number of stages in the shift register section 102 increases more and more, since a higher-resolution image display device with a larger display screen is in demand. Therefore, a two-way shift register and an image display device that does consume rather less electric power even when the distance between the flip-flops F1 and Fn on the both ends increases have been demanded earnestly.